1. Field of the Invention
The present invention relates to a plastic piston of master cylinder for a hydraulic clutch or brake system mounted on a vehicle.
2. Description of the Prior Art
These is known a master cylinder, for example, as disclosed in Japanese Utility Model Laid-Open Publication No. 59-32557 which is illustrated in FIG. 6.
A cylinder body 1 of a tandem master cylinder comprises a bore portion 2 defined in the cylinder body 1 open at the rear end thereof and closed at the front end thereof, and bosses 3, 4 protruding from a peripheral surface of the cylinder body 1 for connecting with an operating fluid reservoir (not shown). There are slidably accommodated in the bore portion 2 a first piston 6 connected to a spring device 5 and positioned toward the an open end 2A of the bore portion 2, a second piston 7, and a spring 8 disposed between the closed end 2B of the bore portion 2 and the bottom of a bore portion 9 of the second piston 7. The spring device 5 comprises a spring 10 for returning the first piston 6, a bearing or retainer 11 for receiving an end of the spring 10 and brought into contact with an end surface of the first piston 6, a retainer 12 of a cup shape for receiving the other end of the spring 10 and brought into contact with an end surface of the second piston 7, and a bolt 13 extending into the retainer 12 and screwed into the end surface of the first piston 6. The first piston 6 has sliding portions 14A, 14B provided around an outer periphery thereof and disposed at both ends thereof and a hole 15 for engaging with a push rod (not shown) at the central portion of the rear end thereof while the rear end is brought into contact with a stopper 18. A liquid supply chamber 16 is defined in the bore portion 2 around the outer periphery of the first piston 6 between the sliding portion 14A and a flange 14a provided at the portion spaced from the sliding portion 14A while the liquid supply chamber 16 communicates with the inside of the boss 4 via a liquid supply hole 19. A main pressure chamber 17 is defined between the first piston 6 and the second piston 7 and communicates with the boss 4 via a return hole 21. The second piston 7 has sliding portions 22A, 22B provided around the outer periphery thereof and disposed at the both ends thereof. A liquid supply chamber 23 is defined in the bore portion 2 around the outer periphery of the second piston 7 between the sliding portions 22A, 22B and communicates at all times with the inside of the boss 3 via a liquid supply hole 26 while it communicates with the inside of the boss 3 via a returning hole 27. An auxiliary chamber 24 is defined between the closed end 2B of the bore portion 2 and the second piston 7. The second piston 7 is prevented from being moved further toward the rear end portion thereof by a stopper 25 provided on the cylinder body 1 and projecting into the liquid supply chamber 23. Seal members 28, 29 are respectively provided on a seal mounting portion protruding forwardly from the sliding portion 14A of the first piston 6 and a seal mounting portion defined between the sliding portion 14B and the flange 14a. Seal members 30, 31 are respectively provided on a seal mounting portion protruding from the sliding portion 22A of the second piston 7 and a seal mounting portion provided between the sliding portion 22B and a flange 22b disposed on the portion spaced from the sliding portion 22B.
According to the prior art master cylinder, the first and the second pistons 6, 7 have been molded from metal by a mechanical process. However, the master cylinder of a hydraulic clutch or brake system is proposed to be molded from plastic. Inasmuch as the pistons 6, 7 are integrally molded, it is inevitable that there are produced parting lines on the surface of the piston along guide surfaces of a flash mold, namely extending in the axial direction of the piston. Inasmuch as the parting lines can also be formed on the seal mounting portion wherein the annular seal member or seal ring is mounted, there is a possibility that seal formed between the inner peripheral surface of the seal member or seal ring and the bottom surface of the seal mounting portion will be deteriorated by the parting lines.
The present invention has been made to overcome the problems set forth above and provides a plastic piston obviating the parting lines normally formed on the seal mounting portion.
The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.